1. Steel and martensitic transformation
a–Fe ferrite
BCC
–Fe austenite
FCC
Martensite:
needle crystals of a
form aligned in g form
C is diluted in a form
hard but brittle
New phase:
Fe3C cementite
in a-ferrite
hard and ductile
Fe – 0.5% C alloy at
950OC has g form
quench
to room
temperature
annealing
T<800OC
sel=1000MPa

2. Interaction of dislocation with impurities
elastic
region sL sU
DL/L U L
plastic
deformation
work
hardening
Impurities diffuse to dislocations
and form “clouds”-> dislocations
are pinned -> higher elastic limit
When s exceeds sU dislocations
escapes impurities -> stress needed
for plastic deformation decreases

3. Fracture s s sM s Brittle fracture happens in crystals with
little plastic deformation
In polycrystalline solids fracture surface
follows grain boundaries
In single crystal fracture follows cleavage
plane (e.g. {100} in NaCl) s s x sM xC a
a+x s

4. Grifith cracks stress lines are concentrated fracture at lower
at the end of
the crack
fracture at lower
stress
crack l r
Recipe: composite materials, toughening (superficial quench under
compression)
glass
ductile polymer
glass

5. Ductile fracture s s Metal beyond elastic limit: plastic deformation
followed by ductile fracture
Necking: region of smaller cross- section has
stronger stress; great deformation
at neck, even grains are broken
Ductile-brittle transition happens in metal below
~ 0OC. Mobility of dislocation falls ->
elastic limit increases and becomes
higher than fracture stress
neck s

6. Creep e s T<0.4Tm T>0.4Tm strain time failure linear creep
Plastic deformation at constant
stress (> elastic limit) increases
With time when T>0.4Tm
Creep originates from increased
atomic mobility at high temperature:
dislocation climb, elongation of
grains, polygonization s
elongation of
grains
polygonization

7. High temperature alloys against creep
Superalloys: Ni, Cr with added Al and Ti
no creep even at T>0.4Tm
precipitates of added elements
grains of metal oxide are incorporated
powder metallurgy
single crystal in superalloys